Doctor of Philosophy

dissertationAxons degenerate following injury and during "dying-back" neurodegenerative diseases, which are characterized by early degeneration of synapses and axons. While injury and disease are clearly distinct events, they share some common regulators. Although these problems have been extensively studied, very little is known about which molecules initiate and execute the degeneration process. Uncovering mediators of axonal degeneration could provide new therapeutic targets for the treatment of neurodegenerative disease. This dissertation studies axon degeneration in the nematode Caenorhabditis elegans, where degeneration can be induced genetically or by injury. Axons degenerate in a manner consistent with Wallerian degeneration; they swell and fragment into debris particles that are engulfed by other cells. I have studied two main aspects of axon degeneration. First, I have shown that mitochondria are required to prevent the initiation of axon degeneration. I characterize a novel gene ric-7, which is required for mitochondrial export from neuronal cell bodies. In both ric-7 mutant and wild-type axons, degeneration is enhanced in the absence of mitochondria. Thus, mitochondria primarily protect axons and are not required for the execution of degeneration. Second, I have identified genes required for nonprofessional phagocytes to engulf neuronal debris following degeneration. I demonstrate that hypodermal cells engulf degenerating axons in C. elegans and that only some of the programmed cell death (ced) genes are involved. There are two parallel pathways that mediate engulfment of cell corpses during apoptosis; ced-1, -6, -7 and ced-2, -5, -10, -12. Both ced-1 and ced-6 are required for engulfment of degenerating axons, but not the parallel ced-2, -5, -12 pathway. Thus, epithelial cells engulf degenerating axons in a manner that is distinct from both apoptosis and macrophage/gliamediated engulfment of degenerating axons. C. elegans will be a useful model system for uncovering novel regulators of mitochondria-mediated axon protection and epithelia-mediated engulfment

Axons Degenerate in the Absence of Mitochondria in C. elegans

Many neurodegenerative disorders are associated with mitochondrial defects [1, 2 and 3]. Mitochondria can play an active role in degeneration by releasing reactive oxygen species and apoptotic factors [4, 5, 6 and 7]. Alternatively, mitochondria can protect axons from stress and insults, for example by buffering calcium [8]. Recent studies manipulating mitochondria lend support to both of these models [9, 10, 11, 12 and 13]. Here, we identify a C. elegans mutant, ric-7, in which mitochondria are unable to exit the neuron cell bodies, similar to the kinesin-1/unc-116 mutant. When axons lacking mitochondria are cut with a laser, they rapidly degenerate. Some neurons even spontaneously degenerate in ric-7 mutants. Degeneration can be suppressed by forcing mitochondria into the axons of the mutants. The protective effect of mitochondria is also observed in the wild-type: a majority of axon fragments containing a mitochondrion survive axotomy, whereas those lacking mitochondria degenerate. Thus, mitochondria are not required for axon degeneration and serve a protective role in C. elegans axons